There have been some questions (and answers) that talk about using an airplane as part (or all) of the launch to space. There are active designs that use wings as part of the launch or landing configuration.

Getting to "space" while not easy, it is simple enough that college groups have made serious attempts to reach space But getting to height is not the important part, going fast is. Being in orbit is about going fast enough so that as gravity pulls you to earth you keep missing it. Getting the speed is the hard part. If you have wings providing lift, you don't need to spend energy on lift it can mostly go to gaining speed.

In theory if you fly around the Earth in a plane fast enough you are in orbit. As you approach orbital speed, your wings start being used to provide downward force so you can stay in the atmosphere and gain more speed with your air breathing engine. Once you have reached optimal speed, point the plane up and you are in the orbit your speed will support.

The Space Shuttle weighs in around 2,000 tons with fuel, while the Antonov An-225 Mriya wieghs in around 640 tons fully loaded. It seems probable that with the lighter fuel requirements of an air breathing engine, a plane that could carry enough fuel to get to the same orbital speeds as the space shuttle is plausible.

So what is keeping us from reaching orbit in a plane with an air breathing engine?

$\begingroup$Would be like adding the problems with high speed landing to the problems with launching. Even Skylon doesn't have the idea of reaching mach 25 in the atmosphere, because of the aerodynamic friction heat.$\endgroup$
– LocalFluffDec 10 '16 at 12:03

3

$\begingroup$How will Mriya's engines, designed for <1 Mach, perform at 1.0 Mach? At 2 Mach? At 3 Mach? Take a look at SR-71 well-documented troubles... Then what about 25 Mach, or 8 km/s, which is the orbital speed?$\endgroup$
– kubanczykDec 10 '16 at 12:05

1

$\begingroup$"Once you have reached optimal speed, point the plane up and you are in the orbit your speed will support. .. So what is keeping us from reaching orbit in a plane with an air breathing engine?" Barring all other potential challenges, the craft will be in an elliptical orbit with a periapsis within the (upper reaches of the) atmosphere So I expect there'd only be a single orbit, barring a further propulsive push at the high point to circularise the orbit. Unless of course, the craft gains so much speed as to allow it to exceed the Earth's escape velocity.. Just musing really, please ..$\endgroup$
– Andrew ThompsonDec 10 '16 at 13:32

2

$\begingroup$At hypersonic speeds ordinary jet engines don't work well. Turbo- and ram- jet engines decelerate the incoming flow to subsonic speeds with associated inefficiencies. You need to switch to a scramjet. Unfortunately those are quite hard to get to work, no one has yet succeeded in making one that runs for more than a few seconds. en.wikipedia.org/wiki/Scramjet And you have a long way to go from Mach 5 to orbital speed. Your scramjet needs to run for a long time.$\endgroup$
– Organic MarbleDec 10 '16 at 13:40

1

$\begingroup$"your wings start being used to provide downward force" would not be necessary/relevant. By the time there is enough velocity that a downward aerodynamic force would be needed to keep the vehicle in the atmosphere, orbital velocity has been achieved. The vehicle could be allowed to move out of the atmosphere at which point conventional rockets would be needed to adjust the size and shape of the orbit (such as to increase perigee/circularize).$\endgroup$
– Anthony XDec 20 '16 at 22:10

6 Answers
6

The fastest airbreathing engines we have are scramjets (supersonic combustion ramjet), i.e. a duct that compresses the air while allowing it to flow at supersonic speeds. The fastest scramjet flight so far reached Mach 5, or about 1/5 of orbital speed.

All other airbreathing engines are slower, mainly because they only work when the air going into the engine is slowed to subsonic speeds. The J-85 engines of the SR-71 are just about the limit for that sort of technology, at about Mach 3.5.

The difficulty with a scramjet is igniting the air/fuel mixture at supersonic speeds and keeping it lit. The incoming air constantly tries to extinguish the combustion.

The most promising development in this area is the Reaction Engines SABRE, which is designed to run as an airbreathing engine up to Mach 5, and switch to a pure rocket mode above that.

Getting to orbital speed inside the atmosphere is difficult. At Mach 2, a Concorde is about 1 ft longer than when it's on the ground because the heat from air friction expands its structure. At Mach 3 (1/8 of orbital speed), the airframe of the SR-71 had to be built in titanium because aluminium wouldn't withstand the heat.

So a rocket accelerating to orbital speed is in a race: it has to get out of the atmosphere before air friction melts it. That means you have a short window in which you could use airbreathing engines, after which those engines are dead weight (unless you can keep using them, as in the SABRE concept).

$\begingroup$The SR-71 would also leak like a sieve on the ground, because the expansion of the various parts of the airframe due to friction heating was taken into account, so that it would be only sealed tight by the expanding parts when in cruise. And the pilots would cook their rations by simply holding them against the windshield, which would reach temperatures in excess of 300°C/600°F. And that's going really slow compared to orbital speed.$\endgroup$
– Jörg W MittagDec 11 '16 at 9:06

1

$\begingroup$Plus one for mentioning the heating problem. Even if we could make a scramjet capable of functioning at Mach 25, addressing the heating of the airframe is beyond any reasonably imaginable technology. Reentry vehicles are blunt, which blunts the heating and also helps to slow the vehicle down. A blunt launch vehicle doesn't make sense.$\endgroup$
– David HammenDec 13 '16 at 16:29

2

$\begingroup$@JörgWMittag They also took off with the tanks almost completely empty and immediately did an in air refueling because their lift at take off speed was so low as it was tuned for their high altitude high speed flight.$\endgroup$
– Evan SteinbrennerDec 20 '16 at 20:08

$\begingroup$Combustion would not have to be the source of energy. A nuclear ram jet would not have these issues. It would have others, but...$\endgroup$
– ANoneJul 31 at 16:13

For such a vehicle to succeed, a working scramjet engine is required. Turbojet and ramjet engines must decelerate the incoming flow to subsonic speeds in their combustors, resulting in high drag and inefficiencies. A scramjet (the first two letters of which stands for Supersonic Combustion) does not require this deceleration and therefore skips the drag penalties.

Scramjets do not look like regular engines. The engine for the NASP was basically its entire underside - it was a waverider configuration where the underbelly compressed the flow for use in the engine. While an ingenious design, no one has actually gotten this to work for more than a few seconds.

The entire possibility of airbreathing SSTO hinges on a practical, working scramjet engine. Until this technology is achieved, it cannot be done. This was the major factor in the failure of the NASP.

If we neglect temperature, you can technically have a scramjet that gets enough air by going fast enough, maintaining proper climb rate so that lift, drag and intake air remain roughly constant as speed rises.

This all breaks if we look at how heating works.

$$Ram~Rise=SAT\times0.2\times{M}^2$$

$$RAT=SAT+k\times{Ram~Rise}$$

SAT - Static Air Temperature. As read from immobile thermometer. This will be of order of 200-300K.

M - airspeed in Mach. Orbital speed is around Mach 23.2.

k - Ram coefficient, a shape adjustment factor. It will not play a very big role here.

RAT - the temperature as "experienced" by the craft surface

If you substitute the numbers, our baseline starts somewhere around 25,000 Kelvins. Quite likely more. Ram coefficient may reduce this by an order of magnitude maybe, but we're still exceeding melting temperature of tungsten. In short, our craft will totally overheat and burn up.

$\begingroup$The link you give talks about measurement corrections, and that does make sense. Is that "RAT" a real temperature, or merely a thermometer reading? I strongly suspect it is not a real effect, simply because the formula is physically impossible. The units simply do not add up. Temperatures are Kelvin, speeds are in m/s.$\endgroup$
– MSaltersDec 20 '16 at 16:43

$\begingroup$@MSalters: Thermometer reading doesn't come 'from nothing'. Temperature on macroscopic level is the speed of particles of air on the microscopic level, and if your speed relative to air is high enough, the speed of particles hitting the vehicle rises - just the same as with temperature growth, and all the practical effects are exactly the same as being exposed to air of equivalent static temperature. It doesn't matter if the particles move fast and vessel is immobile, or the vessel moves fast relative to particles, resulting energy transfers as heat just the same.$\endgroup$
– SF.Dec 20 '16 at 22:26

What happens at a reentry? An object with orbital speed enters the atmosphere and the compressed air just before this object gets very hot. If the object is not protected by a well designed and complete heat shield, it is destroyed by the heat very fast. The object will loose its speed very rapidly. But this happens not only when leaving an orbit. If you try to reach orbital speed within the atmosphere, you will need a very good heat shield and a lot of power to overcome the friction within the atmosphere and to reach orbital speed. You may save some oxygen by air breathing, but you will need a lot more fuel to overcome the friction. But also the fuel must be protected by a heat shield. In the end you will need a lot more weight for the extra heat shield and fuel than the total weight of a conventional rocket solution.

We have now thousands of objects in an orbit by using a two stage rocket, leaving the atmosphere as fast as possible and accelerating to final orbital speed outside the atmosphere. But nothing has reached an orbit yet by using a single stage airbreathing vehicle.

Well, another approach is to think of the exhaust velocity of the engines. You would have to be able to push the air backwards faster than it comes in in order to get a net thrust forward. You can get more thrust from the additional fuel you burn up but at that point a rocket engine would be more efficient as the intake air is just slowing you down.

So, in order to get to orbital speeds you'd need about 8km/s exhaust velocity (NOT effective exhaust velocity) from your engine, which is basically impossible. Rocket engine exhaust velocity is easy to calculate from Isp and the highest Isp ever achieved with chemical propulsion is around 542s (for a non-practical tripropellant concept engine) which translates to exhaust velocity of 5.3km/s. 8km/s is almost certainly never going to be achieved with chemical reactions. (It would for example need temperatures of >5000C, which is not containable in large enough quantities for an engine to actually produce thrust for anything practical)

If anybody can find actual exhaust velocity for scramjets or similar, I'm definitely interested, I couldn't find any. I might be wrong with those, I know more of rocket propulsion.

Orbits are elliptic. The last point where your engine can breather air is on that orbit. In theory you could reach orbital velocity from there, with the periapsis above ground level, but at the next turn you'd reenter and the orbit would decay.

The only way to counter this is to add some delta-v after leaving the atmosphere to raise your periapsis to above the atmosphere, actually a relatively small amount (200m/s), but with no air you couldn't.

We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

$\begingroup$This does not add anything not already better presented in existing answers.$\endgroup$
– James JenkinsJul 31 at 14:11

1

$\begingroup$This is a valid point, orbital speed and orbit are not the same thing. Sorry this is downvoted.$\endgroup$
– ANoneJul 31 at 16:12

1

$\begingroup$The answer could use some more details but it brings up a good point that the vehicle will need some sort of non air breathing engine or it will come right back down and reenter (unless it achieved escape velocity, which was not the OP's question)$\endgroup$
– Carlos NJul 31 at 17:08